Twisted light boosts optical communications
The current growth rates of network data traffic are largely being driven by services that dramatically increase internet usage. Video-on-demand, social network and cloud computing services are severely stretching the performance of data centres and could affect the functionality of the network. Up until now, a great number of technical solutions and network architectures have been proposed and investigated to address such issues. These mainly relate to improving the scalability, bandwidth and power consumption of data centre optical links and switches. Through the EU-funded project ROAM, scientists used a rarely exploited property of light - its OAM – to boost the amount of data that can be transmitted over an optical fibre and improve the performance of optical switches in data centres. OAM multiplexing Depending on the field spatial distribution, the OAM beam has a helical or twisted wavefront that can take many integer values. “Unlike conventional communications systems that use photons as ones and zeros to carry data, the OAM of light enables additional data to be encoded on its many helical modes. This means that the OAM has the potential to increase the capacity of communications systems,” notes project coordinator, Prof. Antonella Bogoni. ROAM actively pursued the exploitation of this very special property of light to increase fibre transmission capacity for short-reach high-density links. The main concept for increasing capacity lies on combining two different orthogonal multiplexing techniques: mode-division multiplexing using OAM with wavelength-division multiplexing (WDM). Combining OAM and WDM, the project team successfully demonstrated the potential to transmit multiple independent data channels on the same fibre link. Initially, scientists multiplexed 10 co-axial OAM modes transmitted simultaneously over 16 wavelengths on a fibre of 1 km in length. The multiplexed fibre optic transmission system was therefore able to provide 160 data channels. Research then focused on extending the optical fibre transmission range. “We managed to multiplex together 8 OAM modes over 10 wavelengths each carrying 32 Gbs data stream which is the current state of the art” notes Prof. Bogoni.The project team successfully showed that OAM modes can be transmitted up to 10 km. OAM multiplexing demonstrated full compatibility with legacy technologies, while also carrying a minimised channel encoding overhead. High-performance switches in data centres Traditional data centre electrical interconnections suffer from low data rates, poor flexibility and scalability issues, and increased energy consumption. Optical switching has thus started to gain significant momentum because it can provide enhanced scalability through the exploitation of multiple domains such as wavelength, space and time. The ROAM team worked on the development of a reconfigurable switch based on silicon photonics by manipulating different OAM modes and WDM wavelengths. “Adding an additional switching domain within a multi-layer interconnection network can further increase the scalability of optical switches and decrease power consumption,” explains Prof. Bogoni. At first, the team demonstrated a novel silicon photonic switch that is highly scalable and has low energy consumption. Exploiting a combination of 10 OAM modes and 16 WDM wavelengths at 30 Gbs data rates, the switch total capacity reached 20 Tbs per second, allowing highly scalable traffic aggregation. This switching solution was connected to standard optical fibre arrays to make it fully compatible with current data centre architectures. The development of a chip-scale device that can generate, manipulate and switch between OAM modes means there are several applications that extend beyond optical transmission systems including imaging, quantum and sensing.
Keywords
ROAM, orbital angular momentum (OAM), modes, multiplexing, data centre, wavelength-division multiplexing (WDM), optical fibre, optical switch, scalability, transmission capacity